Some vehicle engine systems utilize both direct in-cylinder fuel injection and port fuel injection. The fuel delivery system may include multiple fuel pumps for providing fuel pressure to the fuel injectors. As one example, a fuel delivery system may include a lower pressure fuel pump (or lift pump) and a higher pressure (or direct injection) fuel pump arranged between the fuel tank and fuel injectors. The high pressure fuel pump may be coupled to the direct injection system upstream of a fuel rail to raise a pressure of the fuel delivered to the engine cylinders through the direct injectors. The high pressure pump may also be powered by a driving cam that is coupled to a crankshaft of the engine. A solenoid activated inlet check valve, or spill valve, may be coupled upstream of the high pressure pump to regulate fuel flow into the pump compression chamber. The spill valve may be energized synchronously to the position of the driving cam or engine angular position.
As fuel is being pumped through the fuel system, an important property is the bulk modulus of the fuel. The bulk modulus of a fluid is a measure of that fluid's resistance to uniform compression. In other words, bulk modulus is the ratio of a change in pressure acting on a volume of the fluid to the fractional change in fluid volume. In internal combustion engines that utilize fuel mixtures, such as a gasoline-ethanol blend, measuring the bulk modulus on-board the vehicle and during engine operation may be an effective method to continuously infer the ratio of gasoline to ethanol in the fuel mixture. Additionally, measuring the bulk modulus of the combusting fuel may be important for fuel systems that utilize liquid injection of propane. As liquid propane may become supercritical, its density may vary significantly, thereby creating a need for its density to be continually known as it fluctuates. When liquid propane enters the supercritical fluid phase, its bulk modulus is directly proportional to its density. In this way, a measure of bulk modulus may be used to determine the density of propane as it enters the supercritical phase.
In one approach to measure the bulk modulus of the fuel using the high pressure pump, shown by Sakai et al. in U.S. Pat. No. 7,007,662, an electronic control unit (ECU) learns the bulk modulus of fuel utilizing the fuel pressure before and after actuation of the high pressure pump. In this method, the ECU calculates the pressure difference while also calculating the amount of fuel actually discharged from the high pressure pump. Using the volume and pressure differences, an equation is employed to find the fuel's bulk modulus. In similar methods, a general procedure is followed that can be implemented in many spark-ignited fuel injection systems. Using a combination of pumping a known volume of fuel into the fuel rail while measuring the pressure rise and injecting out a known fuel volume while measuring the pressure drop, the bulk modulus may be found.
However, the inventors herein have identified potential issues with the approach of U.S. Pat. No. 7,007,662. First, it may be difficult to obtain a usable pressure signal from the pressure sensor while the high pressure pump and/or fuel injectors are actively maintaining fuel flow which may cause pressure waves that affect pressure sensor readings. Furthermore, utilizing a measure of actual pumped fuel volume (from the high pressure pump) or injected into the engine from the injectors may be difficult and yield uncertain results. The common methods for determining the fuel's bulk modulus may not be sufficient during normal operation of the fuel injection system.
Thus in one example, the above issues may be addressed by a method, comprising: adjusting duty cycle of a high pressure pump to measure a bulk modulus of a fuel based on a zero flow function for the high pressure pump, the fuel being pumped through the high pressure pump and the zero flow function based on a change in pump duty cycle relative to a resulting change in fuel rail pressure. In this way, the bulk modulus of the fuel may be continuously and reliably learned (calculated) on-board the vehicle. In other methods for determining bulk modulus that may use pressure sensors to record pressure rises responsive to a volume of pumped fuel, steady pressure signals may be unattainable when the direct injection fuel pump and/or fuel injectors are active. Additionally, measuring a volume of fuel pumped or injected from the injectors may yield uncertain results. Also, the bulk modulus calculation methods explained herein may monitor and analyze data produced by the fuel system while the fuel system is injecting fuel into the engine during normal operation modes. The normal operation modes may include various idling and/or fueling conditions such as fueling the engine via port fuel injection only or vice versa.
Using the flow function to determine the fuel's bulk modulus may involve determining a slope of the flow function. The inventors herein have recognized that the slope is directly proportional to the fuel's bulk modulus. Finding the slope (and flow function) can be accomplished in several ways. For example, while not direct injecting fuel into an engine, a series of pump duty cycles are commanded while determining the responsive fuel rail pressures to form a series of operating points. Those operating points can then be plotted to form a zero flow function to find a slope value that is directly proportional to the bulk modulus.
In a related example, while direct injecting fuel into an engine, a multitude of pump duty cycles are commanded at selected fuel rail pressures along with fractional volume of liquid fuel pumped, forming a series of lines that can be used to find intercepts that correspond to zero flow rate data. The zero flow rate data, a series of operating points at zero flow relating fuel rail pressure and duty cycle, can then be plotted to form a zero flow function to find an offset value that may be used to determine the bulk modulus of the fuel.
It is noted that pump duty cycle refers to controlling the closing of the pump solenoid activated inlet check valve (spill valve). For example, if the spill valve closes coincident with the beginning of the engine compression stroke, the event is referred to as a 100% duty cycle. If the spill valve closes 95% into the compression stroke, the event is referred to as a 5% duty cycle. When a 5% duty cycle is commanded, in effect 95% of the displaced fuel volume is spilled and the remaining 5% is compressed during the compression stroke of the pump piston. Duty cycle is equivalent to spill valve timing, in particular the closing of the spill valve.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.